If the supreme court decides a case based on substantive due process it is determining what

Substantive due process protects an individual's life, liberty, or property against certain government actions regardless of the fairness of the procedures used to implement them.
it limits what the government may do in its legislative and executive capacities. Legislation must be fair and reasonable in content and must further a legitimate governmental objective. 
only when state conduct is arbitrary or shocking, will is violate substantive due process
if a law or governmental action limits a fundamental right, the state must have a legitimate and compelling interest to justify its action
in situations not involving fundamental rights, a law or actions does not violate substantive due process if it rationally relates to any legitimate government purpose.